The invention relates to a head-up display adaptable to equipments of a given type, for example aircraft type, land vehicle type or helmet type, and a process for making a set of head-up displays adapted to such equipments.
The head-up display is an optomechanical device capable of superposing a symbology projected to infinity, in other words sufficiently far away so that a pilot does not need to make any substantial accommodation to see the symbology, onto the external surroundings as seen by the aircraft pilot or the vehicle driver or the person wearing the helmet (hereinafter referred to as the xe2x80x9cpilotxe2x80x9d, for reasons of simplicity). The symbology is sent by a usually monochromatic image source, and for example comprises synthetic images and reticles or images originating from infrared sensors capable of seeing the surroundings even on a particularly dark night.
Conventionally, the head-up display (HUD) comprises an optical element called a combiner, that superposes the external surroundings and the symbology projected to infinity by projection optics. For example, the combiner may be an inclined flat sheet coated with a treatment that is reflecting at the wavelength of the image source emitting the symbology and transparent in the visible at other wavelengths, so that a maximum amount of the flux emitted by the external surroundings can be transmitted. However, if the field of vision necessary to see the symbology is large (for example for night vision), it is preferable to use a xe2x80x9cconvergentxe2x80x9d combiner, in other words that behaves like a convergent mirror (for example, a spherical mirror) at the wavelength of the image source, in order to reduce the size of optical elements forming the projection optics. In this case the projection lens, also called the relay optics, forms an intermediate image of the symbology projected by the combiner to infinity. A convergent combiner is particularly necessary when the optical distance between the pilot""s eye and the relay optics is large, which is the case for example in transport aircraft.
But the use of an off-axis convergent combiner, in other words a combiner with a non-zero angle of inclination with the central observation axis of the pilot, necessarily introduces eccentricity aberrations that have to be corrected to provide the pilot with a satisfactory quality image. These aberrations may be corrected by choosing optical elements in the relay optics, and/or by sizing a specific combiner (for example, aspherical combiner). In general, correction of eccentricity aberrations requires the use of specific optical elements in the relay optics, for example cylindrical, aspherical lenses, prisms or other elements that are not rotationally symmetric.
Since eccentricity aberrations depend on the angle of inclination of the combiner, an optical architecture would have to be resized for each installation of a head-up display in two or more items of equipment of a given type, for example an aircraft, land vehicle or helmet, in order to achieve an acceptable correction level. Thus, for example for different models of aircraft when size problems in the pilot""s cockpit are different, positioning constraints on the relay optics with respect to the pilot""s head are also different. In this case, the optical architecture has to be resized for each aircraft model, to adapt the angle of inclination of the combiner and to correct eccentricity aberrations related to the value of the angle of inclination. This means that the final development costs are very high.
In order to overcome these disadvantages, the invention presents a head-up display adaptable to a set of equipments of a given type, with a modular relay optics comprising a first module with at least one optical element that is fixed regardless of the equipment, and a second module with at least one optical element, the position of which is predefined as a function of the angular position of the combiner. More specifically, the invention relates to a head-up display as defined in claim 1.
The invention also presents a process for making a set of head-up displays adapted to equipments of a given type, as defined in claim 8, that can be used to make head-up displays adapted to equipments with different installation configurations without the need to develop a new optical architecture for each item of equipment, and also maintaining the same level of correction of eccentricity aberrations.